Remember that guy? Yeah, that's still an actual champion in the game.
There's always tales of a Sion rework being in the works, but by now it feels like the rework has been in the works since the beta of the game in 1982. So, just let me share some of my ideas on why he needs a rework and how best to fix him.
First off, his mechanics are all over the place and he suffers from the same build sickness as Warwick and Shaco do. He scales with just about every stat you can get in the game, making it very hard to get into him and also making him feel limited in one way or another, no matter what you build. And with Sion in particular, it's like he has two very different kits thrown into one - the kit of an AP burst magician and the kif of a melee ADC.
I think one of the main design flaws with him is the disconnect between his AP nuking spells and his AA kit. For instance, his W encourages you to avoid taking too much damage while it's up, carefully balancing between staying close enough to your enemy and avoiding unnecessary damage - but his E and R encourage you to stick to your enemy and just hit them mindlessly. Example solution: Make the shield grow with Lifesteal, i.e. while the shield is up it grows with damage done by AAs respectively to your Lifesteal value (in addition to or in place of Lifesteal's inherent life gain). I find something like that to be a much more elegant solution than simply making it scale with AD instead of AP. It can still have the AP scaling, but it shouldn't be higher than 0.3.
His Q just sucks. Noone likes it, not even the one playing Sion. The only fun part about it is that enemies will back off instantly when they feel that you're getting to close. But that can be preserved by replacing it with some kind of effective gap closing mechanic.
His E is alright, but I feel that a champion who is this dependant on AA just needs some persistent CC or good movement speed. Also, the health cost just gets pointless with Lifesteal, making this skill just a free stat boost in a duel that your opponent can't really do much about other than simply not engaging you in a AA duel. So, I propose this: Increase the health cost, but have every successful hit on an enemy champion increase his movement speed similar to Evelynn's W, stacking up to a few times, punishing your opponent for not avoiding you immediately.
His Passive is also weird, but mostly because it's based on luck. The idea is pretty cool though and I'd like it to be preserved instead of replaced for something that helps him close gaps. Maybe something similar to Pantheon's passive, but instead of gaining stacks when doing AAs himself, have it gain stacks from receiving AAs. There's probably a better way to do it, though.
Btw, this is all coming from someone who's been having a lot of fun playing AP Sion. (In Dominion, rushing Wooglet's on him is just outright insane.) But I think it is most likely the devs will be turning him AD-exclusive, so there you go.